Hi folks!

Problem

Graphql-java currently fails the document validation for queries that use nullable types for non-nullable schema field arguments with default values. As per spec:

A notable exception to typical variable type compatibility is allowing a variable definition with a nullable type to be provided to a non‐null location as long as either that variable or that location provides a default value.

Here's a discussion from 2018 where this change to the spec was adopted: graphql/graphql-spec#418.

Motivating example:

Schema:

type Query {
  hello(arg: Boolean! = true)
}

Query:

query Test($var: Boolean) {
  hello(arg: $var)
}

This is expected to pass the validation, but currently fails:
ValidationError{validationErrorType=VariableTypeMismatch, queryPath=[hello], message=Validation error of type VariableTypeMismatch: Variable type 'Boolean' doesn't match expected type 'Boolean!' @ 'hello', locations=[SourceLocation{line=3, column=25}], description='Variable type 'Boolean' doesn't match expected type 'Boolean!''}

Solution

This patch fixes the issue by extracting the schema-defined default value for the argument (if present), and checks that in the VariablesTypeMatcher along with the original check. This also adds a regression test to exercise this fix, similar to the test in the reference GraphQL implementation.